A typical prior art intrusion detection system for monitoring a protected area is described in detail in U.S. Pat. No. 3,438,021 issued Apr. 8, 1969. In this system a pair of fluid filled resilient tubes are buried beneath the surface of the ground and associated pressure transducers develop output signals indicative of pressure changes representing the movement of the intruders. While this system provides a valuable and reliable technique for detecting intruders, it is relatively expensive and difficult to install, and requires treatment of the earth to preserve sensitivity under frozen ground conditions.
Recent efforts to utilize coaxial cables in this buried configuration have also failed due to the lack of sensitivity of the coaxial cables resulting from the tight packaging of the wire within the cable which effectively prevents movement of the conducting elements relative to one another in response to intrusion disturbances. The inherent compactness and tightness of the fabrication of a coaxial cable thus limits the output of the co-axial operation to a low frequency signal thus limiting the sensitivity of such a system and rendering it unacceptable in frozen ground applications.